1. Field of the Invention
The invention relates to a method of resolving mixtures of camptothecin enantiomers and analogs thereof.
2. Discussion of the Background
Camptothecin is a pentacyclic alkaloid initially isolated from the wood and bark of Camptotheca acuminata by Wall et al. (M. E. Wall, M. C. Wani, C. E. Cook, K. H. Palmer, A. T. McPhail, and G. A. Sim, J. Am. Chem. Soc., 94, 388 (1966).
Camptothecin is highly biologically active and displays strong inhibitory activity toward the bio-synthesis of nucleic acids. Additionally, camptothecin exhibits potent anti-tumor activity against experimentally transplanted carcinoma such as leukemia L-1210 in mice or Walker 256 tumor in rats.
Several methods for the synthesis of camptothecin and camptothecin analogs are known. These synthetic methods include (i) methods in which naturally occurring camptothecin is synthetically modified and (ii) totally synthetic methods.
Naturally occurring camptothecin belongs to the 20(S) series of compounds, i.e., the single chiral center in the molecule is in the S-configuration.
Since naturally occurring camptothecin has the S-configuration, synthetic methods which modify naturally occurring camptothecin and which do not affect the naturally-occuring chiral center will produce camptothecin analogs having the S-configuration as well.
There are no known syntheses of pure 20(R)-camptothecin, and analogs thereof. Moreover, most synthetic camptothecin analogs are racemic, i.e., 20RS, and the corresponding pure 20S-analogs are unknown. It is well known that the biological activity of the S and R forms of drugs which are racemates may differ, sometimes one will be active and the other also active, but of a different nature. Alternatively, one of the racemate forms may be active, and the other inactive or more toxic. Hence, the present application which presents for the first time a practical method for the synthesis of pure 20(S)- and 20(R)-camptothecin and analogs thereof will be of great value.
Known synthetic methods for producing camptothecin and camptothecin analogs produce a mixture of 20(R) and 20(S)-camptothecins. In most syntheses a racemic mixture or racemic modification, i.e., a 50/50 mixture of the R and S enantiomers, is produced. Examples of a variety of synthetic routes to camptothecin and camptothecin analogs can be found in the following references: Sci. Sin. (Engl. Ed), 21(1), 87-98 (1978); Fitoterpapia, 45(3), 87-101 (1974); Yakugaku Zashi, 92(6), 743-6 (1972); J. Org. Chem., 40(14), 2140-1 (1975); Hua Hsueh Hsueh Pao, 39(2), 171-8 (1981); J. Chem. Soc., Perkin Trans 1, (5), 1563-8 (1981); Heterocycles, 14(7), 951-3 (1980); J. Amer. Chem. Soc., 94(10), 3631-2 (1972); J. Chem. Soc. D, (7), 404 (1970), J. Med. Chem., 23, 554 (1980) and U.S. Pat. No. 4,031,098. Reviews of synthetic methods can be found in The Alkaloids, Brossi, A., Ed. Academic Press, New York (1983) Vol. XXi, Chapt. 4 and Chem. Heterocycle Compd., 25 753 (1983).
A synthetic method of limited practical utility for preparing 20(S)-camptothecin is disclosed in E. J. Corey, D. E. Crouse, J. E. Anderson, J. Org. Chem., 40, 2140 (1975).
The usefulness of a racemic mixture of camptothecin enantiomers, commonly designated as 20(R,S)-camptothecin is limited in view of the fact that the activity of the 20(R) component is largely unknown; present data indicates it is less active as a cytotoxic or antitumor agent and has unknown toxcity. Thus, it would be advantageous to be able to produce the pure enantiomeric compounds separately. A pharmaceutical or veterinary composition comprising only the naturally occurring enantiomer is clearly preferable since the (R)-enantiomer is thought to be inactive or may have undisclosed adverse biological activity. Additionally, required dosages can be reduced since the undesired (R)-enantiomer is no longer present.
A need exists for a method of producing the (R)-enantiomer in a pure state so that its biological activity can be evaluated and tested. Sufficient quantities of the pure enantiomer are also required for any future treatment regimens involving the (R)-enantiomer.
A need exists, therefore, for a method of producing the enantiomers of camptothecin and camptothecin analogs separately and in a pure form.